NAME

perlmodlib - constructing new Perl modules and finding existing ones

THE PERL MODULE LIBRARY

Many modules are included in the Perl distribution. These are described below, and all end in .pm. You may discover compiled library files (usually ending in .so) or small pieces of modules to be autoloaded (ending in .al); these were automatically generated by the installation process. You may also discover files in the library directory that end in either .pl or .ph. These are old libraries supplied so that old programs that use them still run. The .pl files will all eventually be converted into standard modules, and the .ph files made by h2ph will probably end up as extension modules made by h2xs. (Some .ph values may already be available through the POSIX, Errno, or Fcntl modules.) The pl2pm file in the distribution may help in your conversion, but it's just a mechanical process and therefore far from bulletproof.

Pragmatic Modules

They work somewhat like compiler directives (pragmata) in that they tend to affect the compilation of your program, and thus will usually work well only when used within a use, or no. Most of these are lexically scoped, so an inner BLOCK may countermand them by saying:

no integer;
no strict 'refs';
no warnings;

which lasts until the end of that BLOCK.

Some pragmas are lexically scoped--typically those that affect the $^H hints variable. Others affect the current package instead, like use vars and use subs, which allow you to predeclare a variables or subroutines within a particular file rather than just a block. Such declarations are effective for the entire file for which they were declared. You cannot rescind them with no vars or no subs.

The following pragmas are defined (and have their own documentation).

attributes

Get/set subroutine or variable attributes

attrs

Set/get attributes of a subroutine (deprecated)

autouse

Postpone load of modules until a function is used

base

Establish IS-A relationship with base class at compile time

bigint

Transparent BigInteger support for Perl

bignum

Transparent BigNumber support for Perl

bigrat

Transparent BigNumber/BigRational support for Perl

blib

Use MakeMaker's uninstalled version of a package

bytes

Force byte semantics rather than character semantics

charnames

Define character names for \N{named} string literal escapes

constant

Declare constants

diagnostics

Produce verbose warning diagnostics

encoding

Allows you to write your script in non-ascii or non-utf8

fields

Compile-time class fields

filetest

Control the filetest permission operators

if

use a Perl module if a condition holds

integer

Use integer arithmetic instead of floating point

less

Request less of something from the compiler

lib

Manipulate @INC at compile time

locale

Use and avoid POSIX locales for built-in operations

open

Set default PerlIO layers for input and output

ops

Restrict unsafe operations when compiling

overload

Package for overloading perl operations

re

Alter regular expression behaviour

sigtrap

Enable simple signal handling

sort

Control sort() behaviour

strict

Restrict unsafe constructs

subs

Predeclare sub names

threads

Perl extension allowing use of interpreter based threads from perl

threads::shared

Perl extension for sharing data structures between threads

utf8

Enable/disable UTF-8 (or UTF-EBCDIC) in source code

vars

Predeclare global variable names (obsolete)

vmsish

Control VMS-specific language features

warnings

Control optional warnings

warnings::register

Warnings import function

Standard Modules

Standard, bundled modules are all expected to behave in a well-defined manner with respect to namespace pollution because they use the Exporter module. See their own documentation for details.

It's possible that not all modules listed below are installed on your system. For example, the GDBM_File module will not be installed if you don't have the gdbm library.

AnyDBM_File

Provide framework for multiple DBMs

Attribute::Handlers

Simpler definition of attribute handlers

AutoLoader

Load subroutines only on demand

AutoSplit

Split a package for autoloading

B

The Perl Compiler

B::Asmdata

Autogenerated data about Perl ops, used to generate bytecode

B::Assembler

Assemble Perl bytecode

B::Bblock

Walk basic blocks

B::Bytecode

Perl compiler's bytecode backend

B::C

Perl compiler's C backend

B::CC

Perl compiler's optimized C translation backend

B::Concise

Walk Perl syntax tree, printing concise info about ops

B::Debug

Walk Perl syntax tree, printing debug info about ops

B::Deparse

Perl compiler backend to produce perl code

B::Disassembler

Disassemble Perl bytecode

B::Lint

Perl lint

B::Showlex

Show lexical variables used in functions or files

B::Stackobj

Helper module for CC backend

B::Stash

Show what stashes are loaded

B::Terse

Walk Perl syntax tree, printing terse info about ops

B::Xref

Generates cross reference reports for Perl programs

Benchmark

Benchmark running times of Perl code

ByteLoader

Load byte compiled perl code

CGI

Simple Common Gateway Interface Class

CGI::Apache

Backward compatibility module for CGI.pm

CGI::Carp

CGI routines for writing to the HTTPD (or other) error log

CGI::Cookie

Interface to Netscape Cookies

CGI::Fast

CGI Interface for Fast CGI

CGI::Pretty

Module to produce nicely formatted HTML code

CGI::Push

Simple Interface to Server Push

CGI::Switch

Backward compatibility module for defunct CGI::Switch

CGI::Util

Internal utilities used by CGI module

CPAN

Query, download and build perl modules from CPAN sites

CPAN::FirstTime

Utility for CPAN::Config file Initialization

CPAN::Nox

Wrapper around CPAN.pm without using any XS module

Carp

Warn of errors (from perspective of caller)

Carp::Heavy

No user serviceable parts inside

Class::ISA

Report the search path for a class's ISA tree

Class::Struct

Declare struct-like datatypes as Perl classes

Config

Access Perl configuration information

Cwd

Get pathname of current working directory

DB

Programmatic interface to the Perl debugging API (draft, subject to

DB_File

Perl5 access to Berkeley DB version 1.x

Data::Dumper

Stringified perl data structures, suitable for both printing and eval

Devel::DProf

A Perl code profiler

Devel::PPPort

Perl/Pollution/Portability

Devel::Peek

A data debugging tool for the XS programmer

Devel::SelfStubber

Generate stubs for a SelfLoading module

Digest

Modules that calculate message digests

Digest::MD5

Perl interface to the MD5 Algorithm

Digest::base

Digest base class

DirHandle

Supply object methods for directory handles

Dumpvalue

Provides screen dump of Perl data.

DynaLoader

Dynamically load C libraries into Perl code

Encode

Character encodings

Encode::Alias

Alias definitions to encodings

Encode::Byte

Single Byte Encodings

Encode::CJKConstants

Internally used by Encode::??::ISO_2022_*

Encode::CN

China-based Chinese Encodings

Encode::CN::HZ

Internally used by Encode::CN

Encode::Config

Internally used by Encode

Encode::EBCDIC

EBCDIC Encodings

Encode::Encoder

Object Oriented Encoder

Encode::Encoding

Encode Implementation Base Class

Encode::Guess

Guesses encoding from data

Encode::JP

Japanese Encodings

Encode::JP::H2Z

Internally used by Encode::JP::2022_JP*

Encode::JP::JIS7

Internally used by Encode::JP

Encode::KR

Korean Encodings

Encode::KR::2022_KR

Internally used by Encode::KR

Encode::MIME::Header

MIME 'B' and 'Q' header encoding

Encode::PerlIO

A detailed document on Encode and PerlIO

Encode::Supported

Encodings supported by Encode

Encode::Symbol

Symbol Encodings

Encode::TW

Taiwan-based Chinese Encodings

Encode::Unicode

Various Unicode Transformation Formats

Encode::Unicode::UTF7

UTF-7 encoding

English

Use nice English (or awk) names for ugly punctuation variables

Env

Perl module that imports environment variables as scalars or arrays

Errno

System errno constants

Exporter

Implements default import method for modules

Exporter::Heavy

Exporter guts

ExtUtils::Command

Utilities to replace common UNIX commands in Makefiles etc.

ExtUtils::Command::MM

Commands for the MM's to use in Makefiles

ExtUtils::Constant

Generate XS code to import C header constants

ExtUtils::Embed

Utilities for embedding Perl in C/C++ applications

ExtUtils::Install

Install files from here to there

ExtUtils::Installed

Inventory management of installed modules

ExtUtils::Liblist

Determine libraries to use and how to use them

ExtUtils::MM

OS adjusted ExtUtils::MakeMaker subclass

ExtUtils::MM_Any

Platform-agnostic MM methods

ExtUtils::MM_BeOS

Methods to override UN*X behaviour in ExtUtils::MakeMaker

ExtUtils::MM_Cygwin

Methods to override UN*X behaviour in ExtUtils::MakeMaker

ExtUtils::MM_DOS

DOS specific subclass of ExtUtils::MM_Unix

ExtUtils::MM_MacOS

Methods to override UN*X behaviour in ExtUtils::MakeMaker

ExtUtils::MM_NW5

Methods to override UN*X behaviour in ExtUtils::MakeMaker

ExtUtils::MM_OS2

Methods to override UN*X behaviour in ExtUtils::MakeMaker

ExtUtils::MM_UWIN

U/WIN specific subclass of ExtUtils::MM_Unix

ExtUtils::MM_Unix

Methods used by ExtUtils::MakeMaker

ExtUtils::MM_VMS

Methods to override UN*X behaviour in ExtUtils::MakeMaker

ExtUtils::MM_Win32

Methods to override UN*X behaviour in ExtUtils::MakeMaker

ExtUtils::MM_Win95

Method to customize MakeMaker for Win9X

ExtUtils::MY

ExtUtils::MakeMaker subclass for customization

ExtUtils::MakeMaker

Create a module Makefile

ExtUtils::MakeMaker::FAQ

Frequently Asked Questions About MakeMaker

ExtUtils::MakeMaker::Tutorial

Writing a module with MakeMaker

ExtUtils::MakeMaker::bytes

Version-agnostic bytes.pm

ExtUtils::MakeMaker::vmsish

Platform-agnostic vmsish.pm

ExtUtils::Manifest

Utilities to write and check a MANIFEST file

ExtUtils::Mkbootstrap

Make a bootstrap file for use by DynaLoader

ExtUtils::Mksymlists

Write linker options files for dynamic extension

ExtUtils::Packlist

Manage .packlist files

ExtUtils::testlib

Add blib/* directories to @INC

Fatal

Replace functions with equivalents which succeed or die

Fcntl

Load the C Fcntl.h defines

File::Basename

Split a pathname into pieces

File::CheckTree

Run many filetest checks on a tree

File::Compare

Compare files or filehandles

File::Copy

Copy files or filehandles

File::DosGlob

DOS like globbing and then some

File::Find

Traverse a directory tree.

File::Glob

Perl extension for BSD glob routine

File::Path

Create or remove directory trees

File::Spec

Portably perform operations on file names

File::Spec::Cygwin

Methods for Cygwin file specs

File::Spec::Epoc

Methods for Epoc file specs

File::Spec::Functions

Portably perform operations on file names

File::Spec::Mac

File::Spec for Mac OS (Classic)

File::Spec::OS2

Methods for OS/2 file specs

File::Spec::Unix

File::Spec for Unix, base for other File::Spec modules

File::Spec::VMS

Methods for VMS file specs

File::Spec::Win32

Methods for Win32 file specs

File::Temp

Return name and handle of a temporary file safely

File::stat

By-name interface to Perl's built-in stat() functions

FileCache

Keep more files open than the system permits

FileHandle

Supply object methods for filehandles

Filter::Simple

Simplified source filtering

Filter::Util::Call

Perl Source Filter Utility Module

FindBin

Locate directory of original perl script

GDBM_File

Perl5 access to the gdbm library.

Getopt::Long

Extended processing of command line options

Getopt::Std

Process single-character switches with switch clustering

Hash::Util

A selection of general-utility hash subroutines

I18N::Collate

Compare 8-bit scalar data according to the current locale

I18N::LangTags

Functions for dealing with RFC3066-style language tags

I18N::LangTags::List

Tags and names for human languages

I18N::Langinfo

Query locale information

IO

Load various IO modules

IO::Dir

Supply object methods for directory handles

IO::File

Supply object methods for filehandles

IO::Handle

Supply object methods for I/O handles

IO::Pipe

Supply object methods for pipes

IO::Poll

Object interface to system poll call

IO::Seekable

Supply seek based methods for I/O objects

IO::Select

OO interface to the select system call

IO::Socket

Object interface to socket communications

IO::Socket::INET

Object interface for AF_INET domain sockets

IO::Socket::UNIX

Object interface for AF_UNIX domain sockets

IPC::Open2

Open a process for both reading and writing

IPC::Open3

Open a process for reading, writing, and error handling

IPC::SysV

SysV IPC constants

IPC::SysV::Msg

SysV Msg IPC object class

IPC::SysV::Semaphore

SysV Semaphore IPC object class

List::Util

A selection of general-utility list subroutines

Locale::Constants

Constants for Locale codes

Locale::Country

ISO codes for country identification (ISO 3166)

Locale::Currency

ISO three letter codes for currency identification (ISO 4217)

Locale::Language

ISO two letter codes for language identification (ISO 639)

Locale::Maketext

Framework for localization

Locale::Maketext::TPJ13

Article about software localization

Locale::Script

ISO codes for script identification (ISO 15924)

MIME::Base64

Encoding and decoding of base64 strings

MIME::Base64::QuotedPrint

Encoding and decoding of quoted-printable strings

Math::BigFloat

Arbitrary size floating point math package

Math::BigInt

Arbitrary size integer math package

Math::BigInt::Calc

Pure Perl module to support Math::BigInt

Math::BigRat

Arbitrarily big rationals

Math::Complex

Complex numbers and associated mathematical functions

Math::Trig

Trigonometric functions

Memoize

Make functions faster by trading space for time

Memoize::AnyDBM_File

Glue to provide EXISTS for AnyDBM_File for Storable use

Memoize::Expire

Plug-in module for automatic expiration of memoized values

Memoize::ExpireFile

Test for Memoize expiration semantics

Memoize::ExpireTest

Test for Memoize expiration semantics

Memoize::NDBM_File

Glue to provide EXISTS for NDBM_File for Storable use

Memoize::SDBM_File

Glue to provide EXISTS for SDBM_File for Storable use

Memoize::Storable

Store Memoized data in Storable database

NDBM_File

Tied access to ndbm files

NEXT

Provide a pseudo-class NEXT (et al) that allows method redispatch

Net::Cmd

Network Command class (as used by FTP, SMTP etc)

Net::Config

Local configuration data for libnet

Net::Domain

Attempt to evaluate the current host's internet name and domain

Net::FTP

FTP Client class

Net::NNTP

NNTP Client class

Net::Netrc

OO interface to users netrc file

Net::POP3

Post Office Protocol 3 Client class (RFC1939)

Net::Ping

Check a remote host for reachability

Net::SMTP

Simple Mail Transfer Protocol Client

Net::Time

Time and daytime network client interface

Net::hostent

By-name interface to Perl's built-in gethost*() functions

Net::libnetFAQ

Libnet Frequently Asked Questions

Net::netent

By-name interface to Perl's built-in getnet*() functions

Net::protoent

By-name interface to Perl's built-in getproto*() functions

Net::servent

By-name interface to Perl's built-in getserv*() functions

O

Generic interface to Perl Compiler backends

ODBM_File

Tied access to odbm files

Opcode

Disable named opcodes when compiling perl code

POSIX

Perl interface to IEEE Std 1003.1

PerlIO

On demand loader for PerlIO layers and root of PerlIO::* name space

PerlIO::encoding

Encoding layer

PerlIO::scalar

In-memory IO, scalar IO

PerlIO::via

Helper class for PerlIO layers implemented in perl

PerlIO::via::QuotedPrint

PerlIO layer for quoted-printable strings

Pod::Checker

Check pod documents for syntax errors

Pod::Find

Find POD documents in directory trees

Pod::Functions

Group Perl's functions a la perlfunc.pod

Pod::Html

Module to convert pod files to HTML

Pod::InputObjects

Objects representing POD input paragraphs, commands, etc.

Pod::LaTeX

Convert Pod data to formatted Latex

Pod::Man

Convert POD data to formatted *roff input

Pod::ParseLink

Parse an L<> formatting code in POD text

Pod::ParseUtils

Helpers for POD parsing and conversion

Pod::Parser

Base class for creating POD filters and translators

Pod::Perldoc::ToChecker

Let Perldoc check Pod for errors

Pod::Perldoc::ToMan

Let Perldoc render Pod as man pages

Pod::Perldoc::ToNroff

Let Perldoc convert Pod to nroff

Pod::Perldoc::ToPod

Let Perldoc render Pod as ... Pod!

Pod::Perldoc::ToRtf

Let Perldoc render Pod as RTF

Pod::Perldoc::ToText

Let Perldoc render Pod as plaintext

Pod::Perldoc::ToTk

Let Perldoc use Tk::Pod to render Pod

Pod::Perldoc::ToXml

Let Perldoc render Pod as XML

Pod::PlainText

Convert POD data to formatted ASCII text

Pod::Plainer

Perl extension for converting Pod to old style Pod.

Pod::Select

Extract selected sections of POD from input

Pod::Text

Convert POD data to formatted ASCII text

Pod::Text::Color

Convert POD data to formatted color ASCII text

Pod::Text::Overstrike

Convert POD data to formatted overstrike text

Pod::Text::Termcap

Convert POD data to ASCII text with format escapes

Pod::Usage

Print a usage message from embedded pod documentation

SDBM_File

Tied access to sdbm files

Safe

Compile and execute code in restricted compartments

Scalar::Util

A selection of general-utility scalar subroutines

Search::Dict

Search for key in dictionary file

SelectSaver

Save and restore selected file handle

SelfLoader

Load functions only on demand

Shell

Run shell commands transparently within perl

Socket

Load the C socket.h defines and structure manipulators

Storable

Persistence for Perl data structures

Switch

A switch statement for Perl

Symbol

Manipulate Perl symbols and their names

Sys::Hostname

Try every conceivable way to get hostname

Sys::Syslog

Perl interface to the UNIX syslog(3) calls

Term::ANSIColor

Color screen output using ANSI escape sequences

Term::Cap

Perl termcap interface

Term::Complete

Perl word completion module

Term::ReadLine

Perl interface to various readline packages.

Test

Provides a simple framework for writing test scripts

Test::Builder

Backend for building test libraries

Test::Harness

Run Perl standard test scripts with statistics

Test::Harness::Assert

Simple assert

Test::Harness::Iterator

Internal Test::Harness Iterator

Test::Harness::Straps

Detailed analysis of test results

Test::More

Yet another framework for writing test scripts

Test::Simple

Basic utilities for writing tests.

Test::Tutorial

A tutorial about writing really basic tests

Text::Abbrev

Create an abbreviation table from a list

Text::Balanced

Extract delimited text sequences from strings.

Text::ParseWords

Parse text into an array of tokens or array of arrays

Text::Soundex

Implementation of the Soundex Algorithm as Described by Knuth

Text::Tabs

Expand and unexpand tabs per the unix expand(1) and unexpand(1)

Text::Wrap

Line wrapping to form simple paragraphs

Thread

Manipulate threads in Perl (for old code only)

Thread::Queue

Thread-safe queues

Thread::Semaphore

Thread-safe semaphores

Thread::Signal

Start a thread which runs signal handlers reliably (for old code)

Thread::Specific

Thread-specific keys

Tie::Array

Base class for tied arrays

Tie::File

Access the lines of a disk file via a Perl array

Tie::Handle

Base class definitions for tied handles

Tie::Hash

Base class definitions for tied hashes

Tie::Memoize

Add data to hash when needed

Tie::RefHash

Use references as hash keys

Tie::Scalar

Base class definitions for tied scalars

Tie::SubstrHash

Fixed-table-size, fixed-key-length hashing

Time::HiRes

High resolution alarm, sleep, gettimeofday, interval timers

Time::Local

Efficiently compute time from local and GMT time

Time::gmtime

By-name interface to Perl's built-in gmtime() function

Time::localtime

By-name interface to Perl's built-in localtime() function

Time::tm

Internal object used by Time::gmtime and Time::localtime

UNIVERSAL

Base class for ALL classes (blessed references)

Unicode::Collate

Unicode Collation Algorithm

Unicode::Normalize

Unicode Normalization Forms

Unicode::UCD

Unicode character database

User::grent

By-name interface to Perl's built-in getgr*() functions

User::pwent

By-name interface to Perl's built-in getpw*() functions

Win32

Interfaces to some Win32 API Functions

XS::APItest

Test the perl C API

XS::Typemap

Module to test the XS typemaps distributed with perl

XSLoader

Dynamically load C libraries into Perl code

To find out all modules installed on your system, including those without documentation or outside the standard release, just use the following command (under the default win32 shell, double quotes should be used instead of single quotes).

(The -T is here to prevent '.' from being listed in @INC.) They should all have their own documentation installed and accessible via your system man(1) command. If you do not have a find program, you can use the Perl find2perl program instead, which generates Perl code as output you can run through perl. If you have a man program but it doesn't find your modules, you'll have to fix your manpath. See perl for details. If you have no system man command, you might try the perldoc program.

Note also that the command perldoc perllocal gives you a (possibly incomplete) list of the modules that have been further installed on your system. (The perllocal.pod file is updated by the standard MakeMaker install process.)

Extension Modules

Extension modules are written in C (or a mix of Perl and C). They are usually dynamically loaded into Perl if and when you need them, but may also be linked in statically. Supported extension modules include Socket, Fcntl, and POSIX.

Many popular C extension modules do not come bundled (at least, not completely) due to their sizes, volatility, or simply lack of time for adequate testing and configuration across the multitude of platforms on which Perl was beta-tested. You are encouraged to look for them on CPAN (described below), or using web search engines like Alta Vista or Google.

CPAN

CPAN stands for Comprehensive Perl Archive Network; it's a globally replicated trove of Perl materials, including documentation, style guides, tricks and traps, alternate ports to non-Unix systems and occasional binary distributions for these. Search engines for CPAN can be found at http://www.cpan.org/

Most importantly, CPAN includes around a thousand unbundled modules, some of which require a C compiler to build. Major categories of modules are:

Language Extensions and Documentation Tools

Development Support

Operating System Interfaces

Networking, Device Control (modems) and InterProcess Communication

Data Types and Data Type Utilities

Database Interfaces

User Interfaces

Interfaces to / Emulations of Other Programming Languages

File Names, File Systems and File Locking (see also File Handles)

String Processing, Language Text Processing, Parsing, and Searching

Option, Argument, Parameter, and Configuration File Processing

Internationalization and Locale

Authentication, Security, and Encryption

World Wide Web, HTML, HTTP, CGI, MIME

Server and Daemon Utilities

Archiving and Compression

Images, Pixmap and Bitmap Manipulation, Drawing, and Graphing

Mail and Usenet News

Control Flow Utilities (callbacks and exceptions etc)

File Handle and Input/Output Stream Utilities

Miscellaneous Modules

The list of the registered CPAN sites as of this writing follows. Please note that the sorting order is alphabetical on fields:

Continent | |-->Country | |-->[state/province] | |-->ftp | |-->[http]

and thus the North American servers happen to be listed between the European and the South American sites.

For an up-to-date listing of CPAN sites, see http://www.cpan.org/SITES or ftp://www.cpan.org/SITES .

Modules: Creation, Use, and Abuse

(The following section is borrowed directly from Tim Bunce's modules file, available at your nearest CPAN site.)

Perl implements a class using a package, but the presence of a package doesn't imply the presence of a class. A package is just a namespace. A class is a package that provides subroutines that can be used as methods. A method is just a subroutine that expects, as its first argument, either the name of a package (for "static" methods), or a reference to something (for "virtual" methods).

A module is a file that (by convention) provides a class of the same name (sans the .pm), plus an import method in that class that can be called to fetch exported symbols. This module may implement some of its methods by loading dynamic C or C++ objects, but that should be totally transparent to the user of the module. Likewise, the module might set up an AUTOLOAD function to slurp in subroutine definitions on demand, but this is also transparent. Only the .pm file is required to exist. See perlsub, perltoot, and AutoLoader for details about the AUTOLOAD mechanism.

Guidelines for Module Creation

Do similar modules already exist in some form?

If so, please try to reuse the existing modules either in whole or by inheriting useful features into a new class. If this is not practical try to get together with the module authors to work on extending or enhancing the functionality of the existing modules. A perfect example is the plethora of packages in perl4 for dealing with command line options.

If you are writing a module to expand an already existing set of modules, please coordinate with the author of the package. It helps if you follow the same naming scheme and module interaction scheme as the original author.

Try to design the new module to be easy to extend and reuse.

Try to use warnings; (or use warnings qw(...);). Remember that you can add no warnings qw(...); to individual blocks of code that need less warnings.

Use blessed references. Use the two argument form of bless to bless into the class name given as the first parameter of the constructor, e.g.,:

sub new {
my $class = shift;
return bless {}, $class;
}

or even this if you'd like it to be used as either a static or a virtual method.

Pass arrays as references so more parameters can be added later (it's also faster). Convert functions into methods where appropriate. Split large methods into smaller more flexible ones. Inherit methods from other modules if appropriate.

Avoid class name tests like: die "Invalid" unless ref $ref eq 'FOO'. Generally you can delete the eq 'FOO' part with no harm at all. Let the objects look after themselves! Generally, avoid hard-wired class names as far as possible.

Avoid $r->Class::func() where using @ISA=qw(... Class ...) and $r->func() would work (see perlbot for more details).

Use autosplit so little used or newly added functions won't be a burden to programs that don't use them. Add test functions to the module after __END__ either using AutoSplit or by saying:

eval join('',<main::DATA>) || die $@ unless caller();

Does your module pass the 'empty subclass' test? If you say @SUBCLASS::ISA = qw(YOURCLASS); your applications should be able to use SUBCLASS in exactly the same way as YOURCLASS. For example, does your application still work if you change: $obj = new YOURCLASS; into: $obj = new SUBCLASS; ?

Avoid keeping any state information in your packages. It makes it difficult for multiple other packages to use yours. Keep state information in objects.

Always use -w.

Try to use strict; (or use strict qw(...);). Remember that you can add no strict qw(...); to individual blocks of code that need less strictness.

Always use -w.

Follow the guidelines in the perlstyle(1) manual.

Always use -w.

Some simple style guidelines

The perlstyle manual supplied with Perl has many helpful points.

Coding style is a matter of personal taste. Many people evolve their style over several years as they learn what helps them write and maintain good code. Here's one set of assorted suggestions that seem to be widely used by experienced developers:

Use underscores to separate words. It is generally easier to read $var_names_like_this than $VarNamesLikeThis, especially for non-native speakers of English. It's also a simple rule that works consistently with VAR_NAMES_LIKE_THIS.

Package/Module names are an exception to this rule. Perl informally reserves lowercase module names for 'pragma' modules like integer and strict. Other modules normally begin with a capital letter and use mixed case with no underscores (need to be short and portable).

You may find it helpful to use letter case to indicate the scope or nature of a variable. For example:

Function and method names seem to work best as all lowercase. e.g., $obj->as_string().

You can use a leading underscore to indicate that a variable or function should not be used outside the package that defined it.

Select what to export.

Do NOT export method names!

Do NOT export anything else by default without a good reason!

Exports pollute the namespace of the module user. If you must export try to use @EXPORT_OK in preference to @EXPORT and avoid short or common names to reduce the risk of name clashes.

Generally anything not exported is still accessible from outside the module using the ModuleName::item_name (or $blessed_ref->method) syntax. By convention you can use a leading underscore on names to indicate informally that they are 'internal' and not for public use.

(It is actually possible to get private functions by saying: my $subref = sub { ... }; &$subref;. But there's no way to call that directly as a method, because a method must have a name in the symbol table.)

As a general rule, if the module is trying to be object oriented then export nothing. If it's just a collection of functions then @EXPORT_OK anything but use @EXPORT with caution.

Select a name for the module.

This name should be as descriptive, accurate, and complete as possible. Avoid any risk of ambiguity. Always try to use two or more whole words. Generally the name should reflect what is special about what the module does rather than how it does it. Please use nested module names to group informally or categorize a module. There should be a very good reason for a module not to have a nested name. Module names should begin with a capital letter.

Having 57 modules all called Sort will not make life easy for anyone (though having 23 called Sort::Quick is only marginally better :-). Imagine someone trying to install your module alongside many others. If in any doubt ask for suggestions in comp.lang.perl.misc.

If you are developing a suite of related modules/classes it's good practice to use nested classes with a common prefix as this will avoid namespace clashes. For example: Xyz::Control, Xyz::View, Xyz::Model etc. Use the modules in this list as a naming guide.

If adding a new module to a set, follow the original author's standards for naming modules and the interface to methods in those modules.

If developing modules for private internal or project specific use, that will never be released to the public, then you should ensure that their names will not clash with any future public module. You can do this either by using the reserved Local::* category or by using a category name that includes an underscore like Foo_Corp::*.

To be portable each component of a module name should be limited to 11 characters. If it might be used on MS-DOS then try to ensure each is unique in the first 8 characters. Nested modules make this easier.

Have you got it right?

How do you know that you've made the right decisions? Have you picked an interface design that will cause problems later? Have you picked the most appropriate name? Do you have any questions?

The best way to know for sure, and pick up many helpful suggestions, is to ask someone who knows. Comp.lang.perl.misc is read by just about all the people who develop modules and it's the best place to ask.

All you need to do is post a short summary of the module, its purpose and interfaces. A few lines on each of the main methods is probably enough. (If you post the whole module it might be ignored by busy people - generally the very people you want to read it!)

Don't worry about posting if you can't say when the module will be ready - just say so in the message. It might be worth inviting others to help you, they may be able to complete it for you!

README and other Additional Files.

It's well known that software developers usually fully document the software they write. If, however, the world is in urgent need of your software and there is not enough time to write the full documentation please at least provide a README file containing:

A description of the module/package/extension etc.

A copyright notice - see below.

Prerequisites - what else you may need to have.

How to build it - possible changes to Makefile.PL etc.

How to install it.

Recent changes in this release, especially incompatibilities

Changes / enhancements you plan to make in the future.

If the README file seems to be getting too large you may wish to split out some of the sections into separate files: INSTALL, Copying, ToDo etc.

Adding a Copyright Notice.

How you choose to license your work is a personal decision. The general mechanism is to assert your Copyright and then make a declaration of how others may copy/use/modify your work.

Perl, for example, is supplied with two types of licence: The GNU GPL and The Artistic Licence (see the files README, Copying, and Artistic, or perlgpl and perlartistic). Larry has good reasons for NOT just using the GNU GPL.

My personal recommendation, out of respect for Larry, Perl, and the Perl community at large is to state something simply like:

Copyright (c) 1995 Your Name. All rights reserved.
This program is free software; you can redistribute it and/or
modify it under the same terms as Perl itself.

This statement should at least appear in the README file. You may also wish to include it in a Copying file and your source files. Remember to include the other words in addition to the Copyright.

Give the module a version/issue/release number.

To be fully compatible with the Exporter and MakeMaker modules you should store your module's version number in a non-my package variable called $VERSION. This should be a floating point number with at least two digits after the decimal (i.e., hundredths, e.g, $VERSION = "0.01"). Don't use a "1.3.2" style version. See Exporter for details.

It may be handy to add a function or method to retrieve the number. Use the number in announcements and archive file names when releasing the module (ModuleName-1.02.tar.Z). See perldoc ExtUtils::MakeMaker.pm for details.

How to release and distribute a module.

It's good idea to post an announcement of the availability of your module (or the module itself if small) to the comp.lang.perl.announce Usenet newsgroup. This will at least ensure very wide once-off distribution.

If possible, register the module with CPAN. You should include details of its location in your announcement.

Some notes about ftp archives: Please use a long descriptive file name that includes the version number. Most incoming directories will not be readable/listable, i.e., you won't be able to see your file after uploading it. Remember to send your email notification message as soon as possible after uploading else your file may get deleted automatically. Allow time for the file to be processed and/or check the file has been processed before announcing its location.

By using the WWW interface you can ask the Upload Server to mirror your modules from your ftp or WWW site into your own directory on CPAN!

Please remember to send me an updated entry for the Module list!

Take care when changing a released module.

Always strive to remain compatible with previous released versions. Otherwise try to add a mechanism to revert to the old behavior if people rely on it. Document incompatible changes.

Guidelines for Converting Perl 4 Library Scripts into Modules

There is no requirement to convert anything.

If it ain't broke, don't fix it! Perl 4 library scripts should continue to work with no problems. You may need to make some minor changes (like escaping non-array @'s in double quoted strings) but there is no need to convert a .pl file into a Module for just that.

Consider the implications.

All Perl applications that make use of the script will need to be changed (slightly) if the script is converted into a module. Is it worth it unless you plan to make other changes at the same time?

Make the most of the opportunity.

If you are going to convert the script to a module you can use the opportunity to redesign the interface. The guidelines for module creation above include many of the issues you should consider.

The pl2pm utility will get you started.

This utility will read *.pl files (given as parameters) and write corresponding *.pm files. The pl2pm utilities does the following:

Adds the standard Module prologue lines

Converts package specifiers from ' to ::

Converts die(...) to croak(...)

Several other minor changes

Being a mechanical process pl2pm is not bullet proof. The converted code will need careful checking, especially any package statements. Don't delete the original .pl file till the new .pm one works!

Guidelines for Reusing Application Code

Complete applications rarely belong in the Perl Module Library.

Many applications contain some Perl code that could be reused.

Help save the world! Share your code in a form that makes it easy to reuse.

Break-out the reusable code into one or more separate module files.

Take the opportunity to reconsider and redesign the interfaces.

In some cases the 'application' can then be reduced to a small

fragment of code built on top of the reusable modules. In these cases the application could invoked as:

NOTE

Perl does not enforce private and public parts of its modules as you may have been used to in other languages like C++, Ada, or Modula-17. Perl doesn't have an infatuation with enforced privacy. It would prefer that you stayed out of its living room because you weren't invited, not because it has a shotgun.

The module and its user have a contract, part of which is common law, and part of which is "written". Part of the common law contract is that a module doesn't pollute any namespace it wasn't asked to. The written contract for the module (A.K.A. documentation) may make other provisions. But then you know when you use RedefineTheWorld that you're redefining the world and willing to take the consequences.